The present invention relates to a vacuum brake power booster of a type comprising a booster housing which is sealingly subdivided into a vacuum chamber and a working chamber by an axially movable wall. A control valve is mechanically operable by a brake pedal actuated valve piston to controllably connect the working chamber to the vacuum chamber or to the atmosphere. The axially movable control housing incorporates evacuation ducts and ventilation ducts which permit evacuation and ventilation to the atmosphere of the working chamber. The movement of the valve piston in the direction opposite to the actuating direction is limited by a U-shaped element which is vertically insertable into the control housing, and the preferably diametrally oppositely extending evacuation ducts lie radially outside of the area between the arms of the U-shaped element.
A vacuum brake power booster of this type is known from the published international application No. WO 91/02670. The element limiting the axial return movement of the valve piston in the control housing is arranged in the exit area of the ventilation ducts in the known brake power booster, which exit area lies within a space bounded by the arms of the U-shaped element. The air flow introduced into the working chamber on actuation is restricted thereby, the response times of the known brake power booster being impaired as a result. What is also disadvantageous are the air turbulences forming in this area which cause disturbing air noises during the actuation.
Likewise the uneven distribution of the plastics material of the control housing is to be considered less favorable, which results in out of round shape during manufacture, and which is due to the evacuation ducts being designed in the control housing and extending radially oppositely outside of the arms of the U-shaped element.
Therefore, it is the object of the present invention to improve upon a vacuum brake power booster of the type described so as to improve air flow and thereby accomplish a shortening of the response time. Further, the noises occurring on actuation are to be reduced to a large extent or entirely eliminated,.